Power supplies may be used for a multitude of purposes and applications. Industrial equipment operating from a high voltage alternating current (ac) supply input often needs a power supply that can provide a regulated low voltage direct current (dc) supply output to power analog and digital circuitry. Such a power supply may convert the high voltage ac into the regulated low voltage dc power usable in applications such as industrial motor drives, uninterruptible power supply (UPS) systems, and energy meters.
One type of power supply is a switching power supply that includes a switch coupled to an energy transfer element. In operation, the switch is utilized to provide the desired output by varying the duty cycle (typically the ratio of the on time of the switch to the total switching period), varying the switching frequency, or varying the number of pulses per unit time of the switch in a switching power supply. The switch is controlled with a power supply control circuit to convert the high voltage ac input into the regulated dc output through the energy transfer element. However, in high voltage applications, such as industrial applications, the switching power supply must be designed to handle high peak input voltages higher than the line voltage provided by the input power source because of line voltage fluctuations. For example, industrial applications typically utilize an input power source of 440 volts (V) ac. After rectification and filtering, the 440 V ac is translated into a direct current voltage of about 625 V dc. With line voltage fluctuations, the power supply should be designed to handle voltages up to 800 V dc.
Typically, the switching power supply includes a clamp circuit coupled across a primary winding of the energy transfer element to limit the voltage across the primary winding to prevent damage to the switch. While the conventional clamp circuit may limit the voltage across the primary winding of the energy transfer element, high voltages may still accumulate across the switch. Thus, when the conventional power supply utilizes a transistor for the switch, the transistor would need to have a high drain-source breakdown voltage (e.g., greater than 1200 volts dc) to handle the high voltages which may accumulate across the transistor. Such high voltage transistors are typically very expensive and add to the cost of the power supply.
In some circumstances, the conventional switching power supply may include an additional clamp circuit coupled to limit the voltage across the switch. One example conventional power supply is shown in U.S. Pat. No. 5,602,724, entitled “Low-Cost, High-Voltage, Flyback Power Supply,” which illustrates in FIG. 2 the use of a transistor 155 to limit a voltage on switch 150. As shown in FIG. 2 of U.S. Pat. No. 5,602,724, power supply 100 further includes a capacitor 114 to filter the input voltage and a separate resistor divider (i.e., resistors 160 and 165) to control transistor 155. However, the use of resistors 160 and 165 adds to the component count and decreases the efficiency of power supply 100 due to dissipation across each resistor, particularly at high input voltages.